Question

biology exam
complete the following punnett square to answer questions 1 - 7.
suppose a female dog is pregnant with a litter of puppies. the mother dog has brown fur and is heterozygous (aa). the father dog has white fur and is homozygous recessive (aa).
complete the punnett square to predict the possible outcome of puppies for this litter

1. name and describe the type of cross shown above. (2pts)
2. what percentage of the offspring will have brown fur?
3. what percentage of the offspring will have white fur?
4. what percentage of the offspring will be heterozygous?
5. what does it mean to be a \carrier\ for a trait? (2pts)
6. suppose the father dog was homozygous dominant (aa). what do you predict the phenotype would be if homozygous dominant?
7. in crosses like this, when is a recessive allele expressed in offspring?

Explanation:

Step1: Complete the Punnett Square

The mother dog has the genotype \(Aa\) (produces gametes \(A\) and \(a\)), and the father dog has the genotype \(aa\) (produces gametes \(a\) and \(a\)).

	\(a\)	\(a\)
\(a\)	\(aa\)	\(aa\)

Step2: Answer Question 1

The cross is a test cross. A test cross is a cross between an individual with an unknown genotype (here the mother dog with brown fur, \(Aa\)) and a homozygous recessive individual (the father dog with white fur, \(aa\)) to determine the unknown genotype.

Step3: Answer Question 2

Looking at the Punnett Square, out of 4 possible offspring genotypes (\(Aa, Aa, aa, aa\)), 2 have the \(Aa\) genotype. Since \(A\) (brown) is dominant, the percentage of offspring with brown fur is \(\frac{2}{4}\times100\% = 50\%\).

Step4: Answer Question 3

The \(aa\) genotype gives white fur. There are 2 \(aa\) genotypes out of 4. So the percentage of offspring with white fur is \(\frac{2}{4}\times100\%=50\%\).

Step5: Answer Question 4

Heterozygous means \(Aa\). There are 2 \(Aa\) genotypes out of 4. So the percentage of heterozygous offspring is \(\frac{2}{4}\times 100\% = 50\%\).

Step6: Answer Question 5

A carrier for a trait is an individual who has one copy of a recessive allele (but does not express the recessive phenotype). For example, in this case, \(Aa\) individuals are carriers for the white - fur trait (since they have the \(a\) allele but show the brown - fur phenotype due to the dominant \(A\) allele).

Step7: Answer Question 6

If the father dog was \(AA\) (homozygous dominant) and the mother dog is \(Aa\):

	\(A\)	\(A\)
\(a\)	\(Aa\)	\(Aa\)

All offspring (\(AA\) or \(Aa\)) would have the dominant phenotype (brown fur), because the dominant \(A\) allele is present in all genotypes.

Step8: Answer Question 7

A recessive allele is expressed in offspring when the offspring has a homozygous recessive genotype (\(aa\)). That is, when an offspring receives a recessive allele from both parents.

Answer:

1. Test cross. A cross between an individual with an unknown genotype and a homozygous recessive individual to determine the unknown genotype.
2. \(50\%\)
3. \(50\%\)
4. \(50\%\)
5. An individual who has one copy of a recessive allele (but does not express the recessive phenotype).
6. All offspring would have brown fur (the dominant phenotype).
7. When the offspring has a homozygous recessive genotype (receives a recessive allele from both parents).